Other Agents

Viruses Some viruses, such as hepatitis A, rotaviruses, and Norwalk virus, can be spread by food as well as by water. In part, this is expected, since water is used in the preparation of many foods. However, unlike bacteria, viruses cannot grow or replicate in food. (Animal viruses only replicate within living animal cells.) Thus, the initial contamination must be with a sufficient quantity of viruses to constitute an infective dose, or disease will not occur.

Fungi Several types of fungi can cause food poisoning. Some wild mushrooms are themselves highly poisonous, but may be mistaken for edible varieties by inexperienced pickers. On the other hand, growth of fungi within a food material can produce toxins. Ergotism, as described in Section 10.7.4, for example, is caused by the neurotoxins produced by the ascomycete Claviceps purpurea in infected cereal grains (especially, rye). Similarly, the aflatoxins produced by some strains of Aspergillus, particularly during growth on stored grains and peanuts, are potential carcinogens (cancer-causing agents). However, infection by ingested fungi is rare.

Algae Some of the marine dinoflagellates occasionally "bloom" in large numbers to produce "red tides" (Section 10.7.2). Certain strains, including many Gymnodinium, Gonyaulax, and Pfisteria, may release neurotoxins that can be taken up by filter-feeding shellfish, such as mussels, clams, and oysters. (Thus, this listing could also be under waterborne disease.) These concentrated toxins, which are not destroyed by cooking, can in turn produce disease in man when ingested, in some cases leading to respiratory failure and death. At least with Pfisteria, there are also reports of skin damage from contact with toxin-contaminated water or fish.

Parasites Toxoplasmosis is caused by the protozoan Toxoplasma gondii, a sporozoan. It is ingested in inadequately cooked meat, using the same utensils for meat before and after cooking, or licking the fingers after handling raw meat. (It can also be spread through contact with cat feces.) In general, infection causes no symptoms, but it can cause birth defects if a pregnant woman becomes infected.

Tapeworms are members of the Cestoda class of flatworms (Platyhelminthes). The most important as human parasites are those acquired by eating raw or undercooked beef, pork, and fish. Encysted larvae in the muscle tissue of these animals are released from the cyst once they are ingested, and attach to the intestinal lining of the small intestine. There they quickly mature, reaching lengths of up to 15 m (50 ft). Tapeworms have no mouth or digestive tract, relying on the host to provide nutrients that it absorbs through its body wall. Over 1 million eggs per day may be produced in the worm's ripened body segments and shed with the human host's feces. If the eggs are ingested by the appropriate intermediate host (cattle, swine, or fish, respectively), the larvae develop and invade the muscle, where they encyst, completing the cycle.

The causative agent of ascariasis is the nematode (roundworm) Ascaris lumbricoides. The ova (eggs) are ingested on contaminated vegetables or from dirty hands, and hatch in the intestine. The larvae penetrate the intestinal wall, migrate to the lungs, climb up the respiratory tract to the throat, and are then swallowed again. The adults now attach to the intestinal wall and feed on the partially digested food, reaching a length of 25 cm (10 in.). After mating, a mature female may produce 200,000 eggs per day. These are shed in the feces and may remain infective in soil or sludge for several months.

Trichinosis is caused by another nematode, Trichinella spiralis. This roundworm is able to infect several species, but humans are exposed mainly by eating undercooked pork (or bear, in some places). The adult matures in the small intestine, with the female producing over 1000 larvae. These migrate throughout the body and then encyst in the muscle. Since human muscle tissue is not eaten, the larvae eventually die, and humans can be considered an accidental host. However, the disease may be very painful and is incurable. Fortunately, trichinosis has been virtually eradicated from the swine herds in the United States.

Prions Prions (Section 10.8.2) are infective proteins that attack the brain of their host. The diseases they cause can be spread through ingestion of infected animal tissue, especially brain, containing the prions, and are always fatal. Cooking is generally considered inadequate to inactivate them. Bovine spongiform encephalitis, or mad cow disease, is probably the most widely known example, and apparently it can affect humans. Scrapie attacks sheep. Two human prion diseases are Creutzfeldt-Jakob disease, which some believe can be transmitted by ingesting beef products from ''mad cows,'' and kuru, which is associated with ritualistic cannibalism among indigenous peoples of New Guinea. Perhaps as a result of the infected cattle herds in Great Britain, 81 people there and two in France have died.

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